1. Field of the Invention
The present invention relates to a semiconductor circuit and particularly to a transistor circuit using a bipolar transistor with high input impedance.
2. Description of the Prior Art
A transistor having its base width greatly reduced in order to improve its high frequency characteristics is disclosed in U.S. Pat. No. 3,591,430.
The above mentioned transistor has such a construction as shown in FIG. 1, which illustrates an NPN-type transistor. In FIG. 1, an embedded base region 10b of, for example, P-type is formed on an N-type semiconductor substrate which is a collector region 10c, and a semiconductor layer forming an emitter region 10e is formed on the base region 10b. On the periphery of the embedded base region 10b there is formed a base electrode contact region 10b' of relatively high impurity concentration in an annular shape which is extended to the semiconductor surface. A base electrode 11b is deposited on the region 10b' in an ohmic contact therewith, an emitter electrode 11e is deposited on the emitter region 10e surrounded by the region 10b' in an ohmic contact therewith, and a collector electrode 11c is deposited on the collector region 10c in an ohmic contact therewith. Reference 10e' designates a region used to deposit the emitter electrode 11e on the emitter region 10e in ohmic contact therewith. Reference characters E, B and C indicate emitter, base and collector terminals, respectively.
In the thus constructed transistor, the portion of the base region facing the semiconductor surface, that is, the portion connected with the base electrode 11b, is formed therein as a region 10b' of relatively high impurity concentration, so that carrier injection is small relative to the surface of the aforesaid portion or relative to the lateral direction. For this reason, the emitter junction is operated mainly at a junction portion j.sub.e between the emitter region and the embedded region 10b, that is, a portion opposite a collector junction j.sub.c. In addition, the thickness of the region 10b is quite small, so that the base transport factor .beta. is high.
Meanwhile, a consideration will be given on the emitter-grounded current amplification factor h.sub.FE which is used as one of parameters for evaluating the characteristics of a transistor. If the base-grounded current amplification factor is taken as .alpha., the emitter-grounded current amplification factor h.sub.FE is given as follows: ##EQU1## Further, the base-grounded current amplification factor .alpha. is given as follows: EQU .alpha. = .alpha.*.beta..gamma. 2.
where .alpha.* is the collector amplification factor, .beta. is the base transport factor, and .gamma. is the emitter injection efficiency.
Considering now an emitter-grounded transistor circuit, a simplified hybrid .pi.-type equivalent circuit for this circuit in a range between middle and high frequencies is shown in FIG. 2. In this circuit, the following letters or characters are employed:
______________________________________ r.sub.x : base resistance, g.pi.: parameter indicating recombination component of base current which is expressed as conduct- ance for the sake of convenience in this specification, C.pi.: parameter indicating component of base current corresponding to variation of excessively stored carriers, C.mu., g.sub.0 : Parameters indicating base width modulation according to collector voltage, g.sub.m : mutual conductance, and C.sub.JE : base-emitter junction capacity. ______________________________________
In this case, if the variation of base current is taken as i.sub.b and the variation of emitter-base voltage as v.sub.be, the parameters g.sub..pi. and C.sub..pi. are defined as follows: ##EQU2##
On the other hand, the current amplification factor h.sub.fe of an AC component is defined by h.sub.fe = I.sub. O /I.sub.i in the case when an output is short-circuited, where I.sub.i is the input current and I.sub.O is the output current. Now, if the complex frequency is taken as s, a voltage V across g.sub..pi. and C.sub..pi. in FIG. 2 is expressed as follows: ##EQU3## Further, since I.sub.O = g.sub.m V and normally C.sub..mu. &lt;&lt; C.sub..pi. , the following relation is obtained: ##EQU4##
Now, in an NPN-type transistor, g.sub..pi. is considered as being divided into a hole current component g.sub.p and an electron current component g.sub.n for the sake of convenience. That is, EQU g.sub..pi. = g.sub.p + g.sub.n 6.
From the equations (5) and (6), the current amplification factor h.sub.FE of DC component or the current amplification factor at a time of s = 0 is expressed as follows: ##EQU5##
In the transistor shown in FIG. 1, since its base transport factor .beta. is made high as described previously, the recombination of electrons is small to satisfy the relation g.sub.n &lt;&lt; g.sub.p. Accordingly, its h.sub.FE is given as follows: ##EQU6## However, in normal transistors including the transistor of this kind, reducing the recombination of holes, is not considered so that g.sub.p is relatively large. For this reason, as apparent from the equation (8), relatively high h.sub.FE can not be obtained.
The conductance components g.sub.p and g.sub.n are expressed as follows. At first, emitter current I.sub.E, collector current I.sub.C and base current I.sub.B are respectively given as follows:
I.sub.E = A(J.sub.n + J.sub.p ) PA1 I.sub.C = .beta.AJ.sub.n PA1 I.sub.B = I.sub.E - I.sub.C PA1 i.sub.b = a{j.sub.p + (1 - .beta.)J.sub.n }
where A is junction area, J.sub.n is current density according to electrons injected from emitter into base, and J.sub.p is current density according to holes injected from base into emitter.
Further, the following relations are obtained: ##EQU7##
A transistor with its J.sub.p being reduced is disclosed in U.S. Pat. No. 2,822,310. In other words, the emitter is provided therein with a high resistive region whose thickness is selected smaller than the diffusion length of minority carriers thereby to reduce the current density J.sub.p according to the diffusion of minority carriers in this emitter.
In this connection, the emitter injection efficiency .gamma. is given as follows: ##EQU8##
Accordingly, the reduction of J.sub.p is equivalent to the enhancement of .gamma..